1. Field of the Invention
The present invention relates generally to systems for producing the soundtrack content for a low-frequency-only channel in a multichannel soundtrack, and more particularly to a method and apparatus for deriving or creating an audio Low Frequency Effect (LFE) signal in 5.1, 6.1, and 7.1 channel sound tracks and musical recordings. The present invention would also relate to systems with more than one low-frequency-only channels
2. Discussion of Related Art
When a multichannel film soundtrack, television program soundtrack, or a musical recording is produced, it is most often done so in a 5.1 channel format. These include five full frequency range channels located respectively in the Front Left, Front Center, Front Right, Surround Right and Surround Left locations of the auditorium or listening room, along with a xe2x80x9c0.1xe2x80x9d low-frequency-only channel generally located along the front of the room. The xe2x80x9c0.1xe2x80x9d channel can be arranged in an electroacoustic dynamic range so as to produce 10 dB higher sound pressure than that of the five main channels for the same modulation of the recording medium.
Using existing technologies, producing the soundtrack content for the 0.1 LFE channel is often a cumbersome and misunderstood process. The LFE channel should theoretically only be used once the low frequency output capabilities of the main channels have been exceeded and the sound recordist would nevertheless desire more low frequency sound pressure level. To achieve the desired effect recordists occasionally send the same signal that is overloading the main channels into the LFE. However, often a different signal is sent to the LFE, and this results in incompatibilities when the soundtrack is played back in 5.1-to-2 channel downmixed mode over a 2 channel playback system. This will happen most often when a multichannel film, television, or music sound recording is reproduced in an end user""s home equipped with standard 2 channel stereo audio system. Standard 2 channel stereo audio systems represent the vast majority of residential sound systems, and compatibility issues must therefore be resolved for proper interchange between production and reproduction spaces. Note that the 2 channel downmix signal is also used by the consumer with a Dolby ProLogic surround decoder, where the downmix would have been done with Dolby Surround Matrix encoding.
What is needed is a means to automatically detect the conditions for main channel overload, and a means for subsequently assigning the overload portion of the signal to the LFE channel.
The Automatic LFE Audio Signal Derivation System of the present invention is well suited for 5.1, 6.1, and 7.1 channel use. For 5.1 channel use, a signal processing device with six audio inputs and six audio outputs would be used to accomplish automatic LFE derivation, said signal processing device incorporating signal measurement functions, signal filtering functions, signal limiting/compressing functions, signal gain adjusting functions, and operating indication functions. Systems for 6.1 and 7.1 channel use would be similar, with the addition of Main channels to the processor.
In operation each main signal fed to the processor is split and processed in three blocks, including a detector, which analyzes the low frequency content of the incoming signal and controls each of two subsequent blocks, including a Variable Shelving Network xe2x80x9cVSNxe2x80x9d block and a Variable Gain Amplifier xe2x80x9cVGAxe2x80x9d block. The VSN is a variable low frequency shelving network in which the amount of low frequency attenuation is variable and which responds to a control signal from the detector circuit. The VGA is a variable gain circuit in which the gain responds to the control signal from the detector circuit. As an incoming signal low frequency level exceeds a threshold programmed into the detector circuit, the VSN attenuates low frequencies and the VGA gain increases from 0X and feeds signals into summing networks leading to a Low Frequency Effect output. With increasing level beyond the above-stated threshold level, the VSN attenuates more low frequency in the main signal output and the VGA feeds more signal into the LFE output. The LFE output is preceded by a Low Pass filter, which has a frequency characteristic matching that of the shelving networks in the VSN. Wherever applicable, the overall Low frequency level is maintained in the listening room by attenuating the overall VGA feeds by 10 dB to compensate for the 10 dB gain in electro-acoustic level of the LFE channel in 5.1, 6.1, and 7.1 sound systems.
Several practical variations on the design of the Variable Shelving Network are set out in the Detailed Description below.